Gas turbine engine components that are subjected to high temperatures are often actively cooled in order to maintain the metal temperature within acceptable limits. Components that partially define a path for the hot combustion gasses are often cooled using impingement cooling of the cooled side and/or film cooling of the hot side. Impingement cooling may be accomplished using a structure with impingement cooling holes designed to direct cooling air onto the cooled side of the component. Manufacturing limitations and design considerations constrain the design of impingement cooling holes. For example, the impingement cooling holes must be sized to permit small particles typically present in the cooling air to pass through without clogging the impingement cooling hole. Additionally, the advantageous effects impingement cooling provides are limited to a relatively small area adjacent the location of impingement. Consequently, many impingement cooling holes are required in order to effectively cool an entire area of the component. Cooling air used for impingement cooling is taken from the gas turbine engine compressor and is redirected away from the combustor to be used in the impingement cooling system. When air is redirected from combustion and used for any other purpose, the engine efficiency is reduced. As a result, increasing the number of impingement cooling holes decreases engine efficiency. Further, the minimum size of the impingement cooling holes required to avoid clogging of the holes often produces a flow volume of impingement cooling air that has a greater capacity to remove heat from the component than is necessary. In other words, a greater volume of cooling fluid may be delivered to the surface to be cooled than is actually required to sufficiently cool the surface. This extra volume of air may not be fully utilized, yet has been taken from the combustor. As a result the combustor operates at reduced efficiency.
Often impingement cooling air is then utilized to provide film cooling on the hot surface of the component via a film cooling hole that delivers the post impingement cooling air to the hot gas path. This film of post-impingement cooling air separates the surface of the component from the hot combustion gasses, and this helps to keep the surface cooler. However, film cooling air may also negatively impact engine performance by slowing the flow of the combustion gasses and by imparting turbulence to the flow (e.g. mixing losses). Any extra volume of cooling fluid in excess of the minimum necessary to sufficiently cool the surface further increases the negative impacts of film cooling on engine performance.
These problems are exacerbated in certain gas turbine engine designs where the combustion gasses are accelerated to approximately mach 0.8 as they exit the combustor, as opposed to conventional designs where this happens upon entering the first stage of the turbine. In such designs, a static pressure difference across the wall of the component that defines the hot gas path is greater than in conventional designs because the hot combustion gasses inside the component are moving much faster. This increased static pressure difference forces more cooling air through the impingement cooling holes than in the conventional design. Further, the greater static pressure difference increases the mixing losses, further reducing engine efficiency. Therefore, there exists a need in the art for improved cooling of components exposed to high operating temperatures.